A schizophrenic patient develops severe muscular rigidity and fever while hospitalized. What is the most likely diagnosis?

The scenario describes a patient with severe muscular rigidity and fever, which are hallmark signs of Neuroleptic Malignant Syndrome (NMS). NMS is a life-threatening condition often associated with antipsychotic medications, particularly typical antipsychotics, but can also occur with atypical medications. It is characterized by a tetrad of symptoms: hyperthermia, muscular rigidity, autonomic dysfunction, and altered mental status. The development of rigidity and fever in a patient who is likely receiving antipsychotic treatment points towards this diagnosis.

In this case, the patient's symptoms align well with NMS, noting that the condition generally occurs after recent initiation or dosage increase of neuroleptic medications. Early recognition is crucial because NMS requires prompt management to prevent further complications.

While meningitis could potentially cause fever and rigidity, it is usually accompanied by neurological signs such as altered mental status and nuchal rigidity, which are not specifically mentioned here. The other options, such as SSRI withdrawal and PCP use, do not typically present with the combination of severe muscular rigidity and fever seen in NMS but rather with different sets of symptoms relevant to those conditions.